As lasers are finding more and more applications in research, industry and medicine among other fields, many uses would benefit from sources of increased power, simplicity or even compactness. However, designing a high power laser oscillator producing the desired spatial (beam quality, fundamental mode operation), temporal (pulse length, pulse repetition frequency) and spectral characteristics (spectral bandwidth) along with stable low-noise operation is often very difficult, if not impossible with the currently available technology.
One solution to realize a laser possessing many of these characteristics and producing a diffraction-limited output beam is to optically end-pump a solid-state laser material. The pump laser diode's output can be transmitted to the crystal through a series of optics and lenses, or delivered to the focusing optics through an optical fiber. The latter solution is often used for it provides a round homogenized beam allowing an optimal pump/laser mode matching. The fiber's output face is then imaged on or close to the front face of the laser medium. The choice of the pump spot's diameter is determined by the size of the mode in the laser medium, as the two should be of about the same size for efficient energy extraction. In turn, the laser mode and pump spot should be kept small enough to match the size of the TEM00 mode in the gain medium, also insuring a high gain, which generally favors the desired output characteristics (low noise, short Q-Switched or mode-locked pulses, lower sensitivity to environmental influences). Lowering the laser medium's gain will increase the lasing threshold, which will generally result in higher noise, longer build-up time and pulse length for Q-switch oscillators and longer pulses for mode-locked oscillators. This choice of pump/mode size therefore limits the amount of pump power that can be applied on a given surface before thermal effects, excessive thermal lensing, bulging of the front surface and ultimately fracture of the medium occur. The gain and the power that can be extracted from the laser medium in a TEM00 beam are therefore limited. One way of increasing this limit is to reduce the pump absorption in the gain medium, spreading the absorption on a longer length. This is usually achieved by reducing the material's doping level. This technique and various embodiments are described in Cheng et al., “Lasers with low doped gain medium”, U.S. Pat. No. 6,185,235. However, the absorption should be adapted so that most of the pump light is absorbed in a region where the pump beam overlaps the laser mode. Unfortunately, the pump laser diode's low beam quality compared to the laser mode limits this absorption to a fraction of the gain medium's length. All the pump radiation that is not absorbed within this short length diverges out of the TEM00 fundamental laser mode and is therefore wasted for the amplification process, potentially allowing high order modes to oscillate. For this reason, the maximum output power available in a diffraction limited beam is limited.
One way of further increasing the output power is through a Master Oscillator Power Amplifier (MOPA) system. It consists of a low or medium power oscillator possessing the desired characteristics previously mentioned (spatial, temporal and spectral characteristics), followed by one or several amplification stages. Those should raise the power of the laser beam, while maintaining spatial, temporal and spectral characteristics. As for a laser oscillator, a limit in applicable pump power arises from the physical limits of the laser medium, before thermally-induced distortions and ultimately the medium's fracture occur. The choice of the pump spot's diameter in the amplifier is determined by the size of the seed beam in the laser medium, as the two should be about the same size for efficient energy extraction. In turn, the seed beam should be kept small enough to reach or approach saturation intensity, insuring efficient extraction of the pump energy. This choice then limits the amount of pump power that can be applied on a given surface before thermal effects, excessive thermal lensing, bulging of the front surface and ultimately fracture of the medium occur. The gain factor and the power that can be extracted from the amplifier are therefore limited. As previously described for an oscillator, the reduction of the pump absorption and therefore the increase in the absorption length allows to further extend the pump power limit before undesirable thermo-optical effects occur. However, the absorption should be adapted so that most of the pump light is absorbed in a region where the pump beam overlaps the seed beam. Unfortunately, the low pump beam quality with regard to the seed beam limits this absorption to a fraction of the gain medium's length. All the pump radiation that is not absorbed within this short length diverges out of the seed beam and is therefore wasted for the amplification process. Consequently, the overall extraction efficiency for a diffraction-limited seed beam is reduced. Also, the undepleated pumped regions may lead to parasitic oscillations or amplified spontaneous emission which would reduce the efficiency of the amplification process.
Such a system has been build, which amplifies a medium power mode-locked oscillator (about 4 W) to a higher output power (42 W), as described in Nebel et al., post deadline paper CPD3, CLEO 1998. Such a high gain factor is achieved through a series of amplification stages. Each of those consist of a Neodymium-Vanadate crystal, which is end-pumped by two fiber-coupled laser diode bars. Although such scheme gives good performance, the gain factor of each stage is limited, thus requiring the use of several amplification stages in series. As the components (fiber coupled diodes, crystals, relaying optics) are expensive, the complexity and the cost of such a system prevents it from being applied to a reliable, cost effective product.
Other systems have been built (Marshall, U.S. Pat. No. 6,144,484), which amplify a seed beam ranging from a power of 100 milliwatts to 10 watts, with a maximum gain factor of two. Undoped end-caps are diffusion bonded to the crystal's ends to act as a thermal reservoir, extracting the heat generated by the absorption of the pump light in the doped region of the crystal, while not absorbing the pump light itself. This leads to a reduced pump-induced bulging of the crystal and a lower temperature of the crystal's pumped volume than with free-standing ends. This results in reduced and less aberrated thermal lensing, ultimately increasing the maximum applicable pump power. However this is just an improvement to classic end-pumping, allowing slightly higher power to be applied to the crystal, but the amplifier's maximum gain factor is still limited to about two. Furthermore the cost and the limited availability of such crystals with diffusion bonded end-caps limits their use in a product.
Yet another system has been build, which amplifies a medium power beam to a high power output, as described in Kafka et al., U.S. Pat. No. 6,417,955. The amplification is achieved in a 12 mm-long, very low-doped (0.15% atm.) Vanadate crystal, that is end pumped by a laser diode stack. The pump light is homogenized and delivered from the diode stack to the crystal through the use of a gold-plated hollow funnel. Pump powers of almost 175 W are applied to the crystal, leading to over 55 W of output power. Although the concept is simple and requires few components, conserving high beam quality along with a high gain factor will be an issue. This is because tight focusing of the pump will be required to achieve high gain along with efficient extraction, leading to thermally induced beam distortions. Furthermore, care must be taken to design or select a pump-light delivery system that forms a homogeneous pump spot, avoiding hot-spots or gain inhomogeneities that would distort the seed beam profile during amplification. Besides, this concept does not apply to lower power systems, much better adapted to fiber-coupled or beam-shaped single bar pump schemes.
Another solution for achieving a high gain amplifier is to multi-pass the seed beam through the gain medium. The seed beam is made bigger than what would be required for efficient extraction in single pass, thus requiring a bigger pump spot and therefore allowing to apply higher pump power. The low intensity seed beam is then passed several times through the pumped region, summing the low extraction of each pass to reach high global extraction efficiency. Such a system is described in Kafka et al., U.S. Pat. No. 5,812,308, including a 1.8 W mode-locked oscillator that is amplified to 6 W in a single end-pumped 4-pass vanadate amplifier. Although a gain factor of over 3 is achieved, scaling the setup to higher power would require reducing the crystal's doping and conversely increasing its length. Thus maintaining a good overlap between the four passes and the pump volume would be difficult to achieve on the whole length of the crystal, while conserving a diffraction limited output beam.
Yet another system has been build, which amplifies a low power source to a medium power output, achieving a high gain factor of over 50. Several setups, including Nd:YAG and Nd:YVO4 based amplifiers are described in Forget et al., Applied Physics B, Lasers and Optics, 2002. The output of a microchip laser of about 100 mW is amplified to over 5 W in a 3D multi-pass scheme. However, such a system is complicated and requires many optical components for obtaining best performance. Furthermore, as with the 4-pass amplifier mentioned above, scaling to higher power and keeping good pump/seed mode matching in a longer crystal would be an issue when a diffraction limited output beam is desired.
Unlike in the approach according to the present invention, conventional end pumping requires high absorption of the pump light close to its input face, before the pump beam's size increases too much due to divergence, thus loosing signal/pump mode-matching. All the pump light absorbed in the volume surrounding the signal mode is therefore wasted and doesn't participate to its amplification when TEM00 operation is desired. Such requirement on high absorption limits the amount of pump power that can be applied to the laser medium for a certain mode size. An increase in pump power will create effects such as thermal lensing, aberrations and bulging of the crystal's front face, effectively distorting the signal beam, and ultimately leading to the crystal's fracture. It is therefore not possible to achieve a high power and high extraction efficiency laser amplifier with such conventional end-pumping technique, while maintaining the high beam quality of a TEM00 or near diffraction-limited beam. Therefore there is a need for a simple high gain and high extraction efficiency gain module concept to be used in a high beam quality laser oscillator or amplifier that could be applied to different signal and pump power ranges. The use of a single high power oscillator in place of a MOPA system, or of a single amplifier in place of multi-stage or multi-pass schemes would allow reducing costs and complexity, thus leading to more competitive and reliable products.
There is a further need for an end-pumping scheme that allows increasing the pump absorption length, and therefore the pump power, while maintaining a good overlap with the seed beam.